1. Field of the Invention
The present invention concerns an x-ray tube of the type having a fixed vacuum housing, in which are arranged an electron-emitting cathode and a ring anode with an impact surface that is struck by the electron beam that is accelerated by an electrical field, as well as with a deflection system to focus and deflect the electron beam. Such x-ray tubes are generally known and serve for the generation of x-ray radiation for examinations of subjects.
2. Description of the Prior Art
Generally, x-ray tubes are used with a fixed anode. In medical technology, for higher outputs the focal spot of the x-ray tube is generated on a focal spot path by rotating the anode by means of an electromagnetic drive, such that the heat load is spread over a large surface. Heat storage ensues by means of a graphite plate. Such x-ray tubes require a complicated positioning of the anode due to the necessary high rotation frequency and (in particular due to the graphite plate) the high weight. Cooling of the anode normally ensues indirectly. Direct cooling is very complicated.
X-ray tubes called rotary piston tubes are known from U.S. Pat. No. 6,292,538, in which the entire housing of the x-ray tube with the anode is rotated while the electron beam is deflected by a deflection system in a direction onto the focal spot path of the anode, such that the x-ray radiation laterally emerges from the x-ray tube at a fixed location. This patent also discloses laterally, discretely, azimuthally deflecting the focal spot on the focal spot path, such that it appears at two positions of the anode. This serves in computed tomography, for example, to increase the resolution, whereby the focus oscillates with high frequency around a half pixel pitch of the detector line (spring focus). In each case the x-ray tube is mechanically rotated in addition to the anode. Also, the rotary piston tube requires a complex positioning of the tube as well as an electromagnetic drive.
Electron beam tomography is known from U.S. Pat. No. 4,962,513, in which the x-ray radiation is generated by a continuously deflected electron beam that is incident on a circular anode arch. The x-ray radiation permeates the measurement field and arrives at a suitably fashioned detector arc. Such an electron beam tomograph requires namely no mechanically moved parts and can be directly cooled, however it has a complicated, large, and expensive assembly, such that it is used only in small numbers.
An x-ray tube is specified in Japanese Application 3 053 436 that is comprised of an electron source, deflection coil, and two electrodes arranged coaxially. An electron beam generated from an electron source is deflected by a deflection coil such that it impacts in the center between the coaxially arranged inner electrode and the outer electrode, and moves circularly on a focal spot path on the inner wall of the outer electrode. A potential difference exists between the electrodes, one underneath the other.